This invention relates generally to communications apparatus and more particularly to an antenna system employing a hybrid transformer to achieve isolation between radio transceivers operating on different radio frequencies.
The present trend in radio communications is to permit two or more colocated parties to communicate simultaneously on separate radio transceivers. For example, several air traffic controllers located within a small control tower must be able to communicate with their respective aircraft. Likewise, an aircraft may employ several different radios and antennas for communicatons which must be operable simultaneously. Still another application involves that of relaying or retransmission of radio signals in which an incoming signal is received at one frequency, demodulated and retransmitted at another frequency.
The communications systems require the use of special antenna systems which are adapted to prevent locally generated strong transmitted radio signals from interferring with simultaneously received weak incoming radio signals. Even if the transmitting and receiving frequencies are widely diverse, interference may still nevertheless be severe. In some cases where sufficient space is available, the transmitting antenna is installed a relatively far distance from the receiving antenna so that interference is diminished to an acceptable level.
In the case of airborne radio systems or in small portable, transportable or vehicular systems, it is not possible to widely space the antennas and consequently it becomes extremely difficult to control and minimize cross-talk interference between colocated transmitters and receivers. Where the wavelength is short, it has been found possible to effect reduced interaction of transmitting and receiving antennas by arranging them in colinear relationship; however, where the wavelength is not short it is necessary to resort to other methods for controlling interference.
One known method for reducing cross-talk interference is by the use of one or more four port hybrid transformers. Such apparatus is taught, for example, in U.S. Pat. No. 3,518,684 entitled "Duplex Antenna Systems for Aircraft Transceivers," H. Brueckmann, June 30, 1970, and U.S. Pat. No. 3,781,684, entitled "Single Antenna Repeater System Utilizing Hybrid Transformers," H.E. Inslerman, Dec. 25, 1973, both patents of which are assigned to the assignee of the present invention.